The invention relates to clock distribution in a communications network and more specifically to clock distribution between a master network node and a slave network node in an asynchronous packet-based communications network.
Traditional telecommunications networks are circuit-switched synchronized networks. Packet-switched networks are typically not synchronized networks. In particular, the well-known Ethernet protocol is an unsynchronized network protocol that transmits traffic in packet bursts. In leading edge packet-switched networks, it is desirable to be able to carry digital voice and data over the same network. Transmitting traffic in bursts in an unsynchronized network is not naturally suited for constant bit rate traffic, such as digital voice traffic, that is sensitive to time delay and jitter. In order to carry constant bit rate (CBR) traffic, such as voice traffic, over an asynchronous packet-based network at an acceptable quality of service (QoS), communications between nodes in the network should be synchronized. Network synchronization techniques involve distributing master clock information from one node of a network to another node of the network. Typically, clock information is distributed from one node to another by transmitting synchronization markers to a slave network node at constant time intervals. In bursty network environments, such as Ethernet networks, it is difficult to distribute clock information at constant time intervals.
In view of the desire to carry digital voice over a network connection using an unsynchronized network protocol such as Ethernet, what is needed is a technique for distributing a clock between nodes of a network connection that can function in a bursty network environment.
Clock information related to a reference clock is distributed from the master network node to the slave network node in an asynchronous packet-based network by embedding the clock information into an additional bit stream and multiplexing the additional bit stream with a primary data stream using an out-of-band channel. The multiplexed stream can then be transmitted from the master network node to the slave network node, where the embedded clock information is demultiplexed from the additional bit stream. In an embodiment, multiplexing the additional bit stream with the primary bit stream using an out-of-band channel involves selecting yB codes of an xB/yB encoded bit stream to represent bits of the additional bit stream or to balance the running disparity of the xB/yB encoded bit stream. The clock information that is embedded into the additional bit stream is used at the slave network node to generate a clock that is synchronized with a reference clock at the master network node. In an embodiment, the clock information represents the time difference between a transmitted frame of the additional bit stream and a next edge of the reference clock. Other data that is carried in the additional bit stream, such as CBR and/or TDM voice data, can then be synchronized to the newly generated clock at the slave network node, thereby creating a synchronized end-to-end connection for the additional bit stream.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.